EP0500562B1 - Circuit arrangement for processing the output from a rev counter - Google Patents

Abstract

Described is a circuit arrangement for processing the output of a rev counter (5) and consisting essentially of a trigger circuit (1, 22) whose switchover points or 'hysteresis' can be controlled and which is fitted with circuits for determining a coupling factor (k) and with circuits for controlling the hysteresis as a function of this coupling factor. The coupling factor (k), multiplied by the rpm or the appropriate sensor-signal frequency, gives the amplitude of the sensor output signal. When the coupling factor (k) is high, the hysteresis curve is tall. When the coupling factor is small, the hysteresis curve is flat.

Description

The invention relates to a circuit arrangement for processing the output signal of a speed sensor, the frequency of which is evaluated to determine the speed, but the amplitude of which is also dependent on the speed. Such speed sensors are required, for example, for anti-lock braking systems (ABS) or for traction control (ASR).

From DE-OS 32 34 637 a circuit arrangement of this type is already known, with which the sensor signals for an electronic anti-lock braking system are processed. With the help of the sensor signals, the information about the wheel turning behavior required for control is obtained. For this purpose, a toothed disc rotates with the wheel, which interacts with a fixed inductive transducer, the output signal of which is in the form of an alternating voltage which is proportional in frequency and amplitude to the wheel speed. With the help of a trigger circuit, the sensor signals are processed, i.e. amplified and converted into a square wave signal or pulse train, the frequency of which corresponds to the speed. The trigger circuit also contains filters with which interference signals are attenuated as far as possible.

The strong dependence of the amplitude on the rotational speed has the consequence that the output signals at low wheel speeds become so weak that they can only be distinguished from the inevitable interference signals by additional measures. Since the voltage induced in the transducer is also significantly dependent on the air gap between the transducer and the toothed disc and thus on installation tolerances, eccentricities of the wheel or the bearing of the wheel, etc., the output signal can assume very different amplitudes at the same wheel speeds and under unfavorable circumstances , ie become extremely weak at low speed and large air gap. This also applies if the mechanical tolerances are kept to a minimum through extensive reworking and adjustment.

A method and a circuit arrangement for processing the sensor output signals have also been developed and described in DE-OS 35 43 058, which have the object of improved separation of the interference signal and the useful signal. For this purpose, two low-pass filters are arranged between the sensor output and the trigger circuit, which generate a useful signal on the one hand and a reference signal on the other hand. The two signals are compared. Depending on the difference between the two signals, a comparator generates a pulse-shaped output signal, namely the processed sensor signal. The reference signal is dynamically tracked with the useful signal with the aid of a control signal, which is obtained by an adaptation circuit.

The strong dependence of the amplitude of the sensor output signal on the speed and on the air gap between the sensor and the toothed disk, however, still complicates the design of such a trigger circuit. The response threshold must take into account the largest air gap, which is permissible in the tolerance range, and be set quite low to the lowest speed to which the controller responds, for example to 100 mV, so that the trigger circuit also responds reliably in this extreme case. In the event of a small or random air gap and a relatively strong interference signal resulting from this, there is then the danger that the trigger threshold will also be reached by this interference signal. The known erasing of the tires over the road, which produces so-called "friction vibrations", dynamic changes in the air gap as a result of road impacts and in many other ways give rise to interference signal levels which are greater than the response threshold which is designed for the lower limit range.

The invention is therefore based on the object of overcoming the disadvantages and developing a circuit arrangement which on the one hand responds very sensitively to useful signals and yet does not react to interference signals of the type described and occurring in practice.

It has been shown that this object can be achieved with a circuit arrangement of the type mentioned, which essentially consists of a flip-flop or trigger circuit whose switchover points or "hysteresis" can be controlled, and with circuits for determining a coupling factor which multiplies by the frequency of the sensor signal corresponding to the speed results in the amplitude of the sensor output signal, and is equipped with circuits for setting the hysteresis of the trigger circuit as a function of the coupling factor.

By automatically calculating the coupling factor and setting the hysteresis accordingly, that the trigger threshold is raised, for example, in the case of an accidentally small air gap which leads to both high useful and relatively high interference signals at the output of the sensor. On the other hand, when there is a large air gap, which weakens the useful signals as well as the interference signals, the trigger threshold is low. To a certain extent, the inevitable installation tolerances of the sensor are compensated for by adapting the trigger circuit or the hysteresis of the trigger circuit. Since the coupling factor can vary in a ratio of 1:20 (ie between 3 and 60 mV / Hz) in an exemplary embodiment, this adaptation is of great importance for the suppression of interference signal effects. The signal-to-noise ratio can be increased by almost the same factor.

The inventive adaptation of the hysteresis to the coupling factor leads to savings on the sensor and on its installation. Compared to previous solutions, higher installation tolerances are permissible. Lower requirements are placed on the signal transmission between the sensor and the electronics because the effect of interference signals is less. The avoidance or considerable reduction of trigger errors leads to an improvement in the control quality of the anti-lock or traction control system.

According to an advantageous embodiment of the invention, the hysteresis of the trigger circuit is matched to the amplitude of the signal which is present at the output of the sensor at the lowest speed to be determined.

Furthermore, the circuit can be designed so that the hysteresis increases continuously or in stages with increasing speed or frequency. The dependency can be chosen in this way be that the distance between the permissible interference level and the useful signal remains approximately the same over the entire speed range.

Another advantageous embodiment of the invention is that the influence of the coupling factor on the increase in hysteresis as a function of the increasing frequency of the sensor signal or the speed of a motor vehicle, based on the occurrence of a predetermined, regularly recurring event, e.g. the actuation of the ignition is weighted or varied. At relatively low speed, the probability that the coupling factor was correctly recognized is lower than at higher speed, which is why the circuit arrangement is expediently designed so that the influence of the coupling factor on the hysteresis increase with increasing speed or frequency of the sensor output signal continuously or increases in several stages. As the vehicle speed decreases, this higher influence on the hysteresis increase remains. Only after the next activation of the ignition is the influence of the coupling factor on the increase in hysteresis gradually or continuously increased.

Further features, advantages and possible uses of the invention will become apparent from the following illustration with reference to the accompanying figures.

Fig. 1

in extremer Vereinfachung das Prinzip der erfindungsgemäßen Schaltungsanordnung und

Fig. 2

im Blockschaltbild ein Ausführungsbeispiel der Schaltung nach Fig. 1 und

Nach Fig. 1 besteht die erfindungsgemäße Schaltungsanordnung im Prinzip aus einer Kippschaltung oder Triggerschaltung 1 mit veränderlicher, steuerbarer Hysterese, aus einem Schaltkreis 2 zur Errechnung des Kopplungsfaktors k und aus einem weiteren Schaltkreis 3 zur Errechnung der Hysterese bzw. der Umschaltpunkte der Triggerschaltung 1 in Abhängigkeit von dem ermittelten, tatsächlich vorhandenen Kopplungsfaktor k und zur Erzeugung eines den Arbeitspunkt und die Hysterese der Triggerschaltung 1 bestimmenden Signals. Mit Hilfe dieser Schaltungsanordnung wird das Ausgangssignal eines Radsensors 5 aufbereitet. Aus diesem Sensorsignal, das in seiner Frequenz und seiner Amplitude von der Raddrehzahl abhängig ist, wird ein Rechtecksignal oder eine Pulsfolge, deren Frequenz oder Pulsabstand ein Maß für die Drehzahl ist, abgeleitet und am Ausgang TA der Schaltung zur Verfügung gestellt. Das Signal am Ausgang TA ist weitgehend von Störungen oder Fehlin£ormationen bereinigt und kann z.B. in der (nicht dargestellten) Reglerelektronik eines ABS weiterverarbeitet werden.Show it

Fig. 1

in extreme simplification the principle of the circuit arrangement according to the invention and

Fig. 2

in the block diagram an embodiment of the circuit of FIG. 1 and

1, the circuit arrangement according to the invention consists in principle of a flip-flop or trigger circuit 1 with variable, controllable hysteresis, a circuit 2 for calculating the coupling factor k and a further circuit 3 for calculating the hysteresis or the switching points of the trigger circuit 1 in dependence from the determined, actually existing coupling factor k and for generating a signal determining the operating point and the hysteresis of the trigger circuit 1. The output signal of a wheel sensor 5 is processed with the aid of this circuit arrangement. A rectangular signal or a pulse train, the frequency or pulse spacing of which is a measure of the speed, is derived from this sensor signal, the frequency and amplitude of which depend on the wheel speed, and made available at the output TA of the circuit. The signal at the TA output is largely cleaned of interference or incorrect information and can be further processed, for example, in the control electronics (not shown) of an ABS.

FIG. 2 shows one possibility of realizing the circuit according to FIG. 1 with the aid of digitally operating circuits.

The available at the output A₁ of the wheel sensor 5 signal of the wheel sensor 5 is first implemented with the aid of a comparator 6, an up-down counter 7 and a digital-to-analog converter 8 in a digital, electronically processable signal. Depending on the state of the output signal (high or low) of the comparator 6, which in turn is determined by the difference between the two input signals of this comparator, the counter 7 counts either upwards or downwards. The work cycle for this counter and for the other construction stages of the circuit shown in FIG. 2, a clock generator 9 is used, the frequency of which, if necessary, can be reduced by a divider 10. In one embodiment of the invention, the clock frequency was 60 kHz and was reduced in step 10 to 30 kHz. The counter reading at output A₃ is compared with the sensor output signal A₁ after conversion into an analog signal.

In addition, the counter reading A₃ over a multiple line 11, e.g. an 8-bit data line, fed to comparators 12, 13 and compared them on the one hand with the stored maximum value (in comparator 12) and on the other hand with the stored minimum value (in comparator 13) of the counter reading. These maximum and minimum values are formed in the memories 14, 15, the inputs of which are also connected to the data line 11. Via an AND gate 16, which is also connected to the work cycle and synchronizes the workflow, the memory content of the maximum memory 14 is increased when the counter reading A at the input of the comparator 12 is greater than the stored one which is fed to the second input B of the comparator 12 Maximum value. The memory content of the minimum memory 15 is corrected accordingly if the value at the input A of the comparator 13 is less than the value at the input B or as the content of the minimum memory 15.

The output signals of the maximum and minimum memories 14 and 15 are fed to an adder 18 in order to determine the mean value of the sensor output signal. The output of the adder 18 leads to an adding / subtracting stage 19 which, for the mean value, represents a "hysteresis" calculated Value is added or subtracted, so that finally a value dependent on the mean and the calculated hysteresis is present at the output of this stage 19, which after conversion into a corresponding analog value using a digital / analog converter 21 is the operating point of a trigger stage 22 that follows the trigger circuit 1 Fig. 1 corresponds. The output signal of the adder 18, which represents the mean value, is also fed back to the memories 14, 15. The second input signal of the trigger stage 22 is the output signal A 1 of the sensor 5. The level at the input A of the trigger circuit 22 is, more precisely, not the operating point but the operating point ± hysteresis. At the output TA of the trigger circuit 22 is the processed signal from the wheel sensor 5 available; it is a square-wave signal that can possibly still be converted into a pulse train.

The maximum and minimum count is also recorded on each positive edge of the output signal TA of the trigger circuit 22 in the memory circuits 23, 24. The amplitude of the sensor signal can then be determined with the aid of a difference generator 25. With each positive edge of the compensation signal TA, the maximum counter reading in the memory 14 is additionally reset to the mean value. With each negative edge, the maximum counter reading in the memory 15 is reset to the mean value. This is necessary to record the current amplitude and the mean.

wobei die Meßfrequenz f bei einer Zähnezahl des (nicht dargestellten) Sensorrades von ungefähr 50 zwischen etwa 30 und 2000 Hz liegt; diese Aufgaben beziehen sich auf einen Radsensor für ein KFZ-Antiblockiersystem.This amplitude of the sensor signal as a function of the speed or the frequency corresponding to this speed enables the coupling factor to be calculated using the formula

${\text{U}}_{\text{sensor}}\text{= kxf,}$

the measurement frequency f for a number of teeth of the (not shown) sensor wheel of approximately 50 lies between approximately 30 and 2000 Hz; these tasks relate to a wheel sensor for a motor vehicle anti-lock braking system.

According to the invention, the hysteresis or the switching points of the trigger circuit 22 (or 1 in FIG. 1) are then determined as a function of this coupling factor.

The sensor amplitude determined in the difference generator 25 is compared and weighted with the aid of a divider circuit 26 with the wheel speed or the frequency corresponding to the wheel speed. The output signal of this divider circuit is recorded in a memory 20 and, as already described, is further processed in stages 19 and 21 to determine the operating point (operating point ± hysteresis). To weight the amplitude determined by the circuit 25 and to determine how high the contribution of the amplitude to the correction of the hysteresis should be, the divider circuit 26 is signaled via a counter 27 and a number of comparators 28 to 30, which speed or which signal frequency meanwhile was achieved. Counter 27 is activated for half a period (10 ms) via a divider 31, which here reduces the frequency of the clock at the output of stage 10 to 5 Hz. During this time, the counter 27 according to FIG. 2 counts the positive edges of the trigger output signal TA. The output signal of the counter 27 is thus a measure of the sensor frequency after completion of the counting process.

If, for example, a lower speed threshold is reached when starting a motor vehicle, a signal according to FIG. 2 corresponds to 40 Hz, the first comparator 28 supplies a corresponding signal to the divider circuit 26. At this relatively low vehicle speed, the measurement of the coupling factor k is still considered to be relatively uncertain. The influence of the currently measured coupling factor or the corresponding amplitude on the correction of the hysteresis is therefore kept relatively low.

As soon as a higher speed is reached, e.g. leads to a signal of 60 Hz, this is signaled by the comparator 29, which leads via the indicated flip-flop and the OR gate to a blocking of the signal coming from the comparator 28 and to driving the input E₃ of the divider circuit 26. The coupling factor k measured at this higher speed is "safer" and its influence on the hysteresis tracking is therefore higher than at the previously described lower speed, which led to a signal at the input E₂. At an even higher speed (120 Hz), the comparator 30 generates the input signal E₄. The coupling factor measured at this speed is weighted highest.

If a higher speed is reached, the hysteresis set by the higher weighting is retained even if the vehicle slows down again. A switch back to the lowest weighting level is made dependent on certain events, for example on the actuation of the ignition. Of course, the type of weighting described and the reset when the ignition is switched off is only one of several expedient options.

The circuit arrangement according to the invention is considerably less sensitive to interference signals because the tipping points or the hysteresis of the trigger circuit no longer has to be set to the worst case - e.g. to the largest air gap between the sensor and the toothed lock washer - but because the hysteresis is automatically increased or the response sensitivity is reduced as much as the coupling factor actually available allows. If the coupling factor is high, both the useful signals and the (induced) interference signals become relatively high. However, increasing the hysteresis prevents the response to these interference signals. On the other hand, if the coupling factor is low, the useful signals become weak; the response sensitivity of the trigger becomes high. However, there is no risk of false triggering because the low coupling factor also weakens the interference signals. The technical progress achieved is therefore considerable.

Claims (7)

1. A circuit configuration for editing the output signal of a speed sensor, e.g. the wheel sensor of an anti-locking control system (ABS) or a traction slip control system (TSC) for automotive vehicles, the frequency of which is analyzed for determining the rotational speed, with the amplitude thereof being, however, equally dependent on the rotational speed, comprising a sweep circuit or a trigger circuit which amplifies the output signal of the speed sensor and converts it into a square-wave signal,
   characterised in that the switch-over points or "hysteresis" of the trigger circuit are controllable, and in that circuits (2) are provided for determining a coupling factor (k) which, multiplied by the frequency of the sensor signal corresponding to the rotational speed, forms the amplitude of the output signal of the sensor, and in that circuits (3) are provided for adjusting the hysteresis of the trigger circuit in response to the coupling factor (k).
2. A circuit configuration as claimed in claim 1,
   characterised in that for determining the coupling factor (k) the amplitude of the signal, which prevails at the output (A₁) of the sensor (5) at the lowest rotational speed to be determined, is determined, and in that the hysteresis of the trigger circuit (1, 22) is adapted to this amplitude.
3. A circuit configuration as claimed in claim 1 or 2,
   characterised in that the hysteresis with an increasing rotational speed and frequency, respectively, rises continuously or in increments.
4. A circuit configuration as claimed in claim 3,
   characterised in that the hysteresis rises with an increasing rotational speed such that the ratio between the permitted noise level and the useful signal remains approximately constant throughout the entire rotational speed range.
5. A circuit configuration as claimed in any one of claims 1 to 4,
   characterised in that the influence of the coupling factor (k) on the hysteresis rise is weighted and varied in response to the frequency of the output signal of the sensor and of the velocity of an automotive vehicle, respectively.
6. A circuit configuration as claimed in claim 5,
   characterised in that the influence of the coupling factor (k) on the hysteresis rise increases with a rising rotational speed continuously or in increments.
7. A circuit configuration as claimed in claim 5 or 6,
   characterised in that the rising influence of the coupling factor (k) on the hysteresis recommences upon the occurrence of a regularly recurrent event, such as the actuation of the ingnition.

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